Blind fastener with frangible nut

ABSTRACT

A blind fastener for connecting a plurality of panels includes a core bolt and a core nut. The core nut surrounds the core bolt. The core nut includes a main body, a nut head, and a tool engagement section. The nut head has a larger outside diameter than the main body. The core nut defines a frangible portion between the nut head and the tool engagement section. The frangible portion is configured to allow the tool engagement section to break off from the nut head when a torque applied to the tool engagement section is greater than a threshold torque after the blind fastener is installed in the plurality of panels.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to and the benefit of U.S. ProvisionalApplication 62/590,915 filed on Nov. 27, 2017. This application alsoclaims priority to and the benefit of U.S. Provisional Application62/631,560 filed on Feb. 16, 2018. This application is also related toco-pending application titled “Two-Piece Blind Fastener,” concurrentlyfiled herewith and commonly assigned with the present application. Thedisclosures of the above applications are incorporated herein byreference.

FIELD

The present disclosure relates generally to fasteners, and moreparticularly to blind fasteners having a core bolt and a sleeve aroundthe core bolt for connecting panels from one side of the panels.

BACKGROUND

The statements in this section merely provide background informationrelated to the present disclosure and may not constitute prior art.

A blind fastener is typically used to secure multiple panels togetherand to be installed from one side (i.e., a front side) of the panels.The blind fastener may include a core bolt and a sleeve surrounding thecore bolt, which are inserted into a hole of the panels. A portion ofthe sleeve adjacent to a rear side of the panel may be deformed duringinstallation of the fastener. The deformed portion of the sleeveprovides a bearing surface to induce preload in the fastener such thatthe panels can be clamped together.

After the deformed portion of the sleeve is formed, the core bolt may berotated to provide a preload to the fastener. When the fastener iscompletely installed, a front portion of the core bolt may break off.The break-off point of the typical core bolt cannot be controlled androtation of the nut relative to the bolt typically needs to becontrolled. In some circumstances it is advantageous to have the fullyinstalled fastener be flush with panels for aesthetics and aerodynamicpurposes. Typical fasteners need to be prepared for painting bypost-installation grinding to be made flush with the panels when thebreak-off point is located outside the countersunk head of the sleeve.

It can be difficult to control rotation of both the bolt and the nut,while also ensuring a flush finished product, maximizing the speed ofinstallation, and reducing cost per fastener. Furthermore, variations ingrip length (i.e., the combined thickness of the panels at the fastener)can occur based on tolerances or design criteria. Accordingly, it isadvantageous for the blind fastener to be able to adapt to variations ingrip length without sacrificing strength of the joint. Moreover, thetypical fastener does not include a torque control feature. Whenexcessive torque is applied to the fastener, the sleeve of the fastenermay flare out to form a tulip configuration, resulting in a defectiveinstallation.

These issues with the installation of blind fasteners are addressed bythe present disclosure.

SUMMARY

In one form, a blind fastener for connecting a plurality of panelsincludes a core bolt and a core nut. The core nut surrounds the corebolt. The core nut includes a main body, a nut head, and a toolengagement section. The nut head has a larger outside diameter than themain body. The core nut defines a frangible portion between the nut headand the tool engagement section. The frangible portion is configured toallow the tool engagement section to break off from the nut head when atorque applied to the tool engagement section is greater than athreshold torque after the blind fastener is installed in the pluralityof panels. In a variety of alternate forms of the present disclosure:the frangible portion is configured to break off from the nut head suchthat an end surface of the nut head is flush with or recessed from afront one of the plurality of panels; a radially outward facing surfaceof the main body of the core nut includes a plurality of dimples; theblind fastener consists of two pieces when in a pre-installed condition,the core bolt being a first one of the two pieces and the core nut beinga second one of the two pieces; the frangible portion includes abreak-off groove that extends along an entire periphery of the core nut;the nut head defines a first cylindrical surface and the tool engagementportion defines a second cylindrical surface, the break-off groove isrecessed radially inward from the first and second cylindrical surfaces;the core nut defines a counter bore having an enlarged part and athreaded part, the enlarged part being formed in the tool engagementportion and having a larger inside diameter than the threaded part,wherein the threaded part is disposed in at least one of the nut headand an end of the main body proximal to the nut head; the main bodydefines a counter bore having a smooth part and a threaded part, thesmooth part having an inside diameter larger than the threaded part suchthat a nut stop shoulder is defined between the smooth part and thethreaded part, wherein the core bolt defines a bolt stop shoulder thatis configured to abut the nut stop shoulder when a frangible portion ofthe core bolt is aligned with or recessed from an outer surface of afront one of the plurality of panels; the core nut further includes asleeve coupled to an end of the main body that is distal to the nuthead, wherein the core bolt includes a bolt head engaged with an end ofthe sleeve that is distal to the main body, and wherein the main bodyincludes a ramp surface configured to slidingly engage an inner surfaceof the sleeve; the bolt head includes a central recess facing axiallyoutward from the core bolt; the sleeve is coupled to the end of the mainbody by a second frangible section of the core nut; the ramp surfaceincludes a plurality of grooves; the nut head defines a conical surfaceand an end surface, the conical surface being configured to abut acountersunk surface of a front panel of the plurality of panels, the endsurface configured to be flush with or recessed from a forward surfaceof the front panel when the conical surface abuts the countersunksurface.

In another form, a blind fastener for connecting a plurality of panelsincludes a core bolt and a core nut. The core bolt includes a first toolengagement section, a threaded section and a bolt head. The threadedsection includes a first frangible portion configured to break thethreaded section when a torque applied to the first handling sectionexceeds a first threshold torque. The threaded section is axiallybetween the first tool engagement section and the bolt head. The corenut includes a main body, a nut head, a second frangible portion, and asecond tool engagement section. The second frangible portion frangiblycouples the second tool engagement section to the nut head and isconfigured to break the second tool engagement section off from the nuthead when a torque applied to the second tool engagement section exceedsa second threshold torque. The nut head is axially between the secondfrangible section and the main body. At least one of the main body andthe nut head defines internal threads configured to engage the threadedsection of the core bolt. The bolt head is configured to engage an endof the core nut that is opposite the nut head. In a variety of alternateforms of the present disclosure: the main body includes a ramp section,a sleeve section, and a third frangible section that frangibly couplesthe sleeve section to the ramp section; the core nut further includes asleeve section and the main body includes a ramp surface configured toslidably engage an inner surface of the sleeve section; the nut headdefines a conical surface and an end surface, the conical surface beingconfigured to abut a countersunk surface of a front panel of theplurality of panels, the end surface configured to be flush with orrecessed from a forward surface of the front panel when the conicalsurface abuts the countersunk surface; the main body defines a counterbore having a smooth part and a threaded part, the threaded partdefining the internal threads, the smooth part having an inside diameterlarger than a major diameter of the internal threads of the threadedpart such that a nut stop shoulder is defined between the smooth partand the threaded part, wherein the core bolt defines a bolt stopshoulder that is configured to abut the nut stop shoulder when afrangible portion of the core bolt is aligned with or recessed from anouter surface of a front one of the plurality of panels.

In still another form, an assembly includes a front panel, a rear panel,and a blind fastener. The front panel defines a front surface. The rearpanel defines a rear surface. The blind fastener is inserted into holesof the front and rear panels for connecting the front and rear panelstogether. The blind fastener includes a core bolt and a core nut. Thecore bolt is received in the holes of the front and rear panels anddefines a break-off notch and a bolt head. The core nut surrounds thecore bolt. The core nut includes a main body, a nut head, a sleeve, anda tool engagement section. The main body extends within the holes of thefront and rear panels. The nut head is disposed in a counterbore orcountersink of the hole of the front panel. The tool engagement sectionis disposed outside the front and rear panels and adjacent to the frontpanel. The sleeve is coupled to an end of the main body that is oppositethe nut head and is disposed outside and adjacent to the rear panel. Thesleeve is configured to be deformed by axial force imparted by the bolthead moving toward the panels. The core nut further defines a break-offgroove that frangibly couples the tool engagement section to the nuthead. The break-off groove of the core nut and the break-off notch ofthe core bolt are flush with or recessed from the front surface of thefront panel when the blind fastener is fully installed. In one form of avariety of alternate forms of the present disclosure, the nut head ofthe core nut includes an end surface that is flush with the frontsurface of the front panel.

Further areas of applicability will become apparent from the descriptionprovided herein. It should be understood that the description andspecific examples are intended for purposes of illustration only and arenot intended to limit the scope of the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure will become more fully understood from thedetailed description and the accompanying drawings, wherein:

FIG. 1 is a cross-sectional view of a blind fastener inserted through acountersunk hole of panels to be connected and constructed in accordancewith the teachings of the present disclosure;

FIG. 2 is a perspective view of a blind fastener constructed inaccordance with the teachings of the present disclosure;

FIG. 3 is a cutaway view of the blind fastener of FIG. 2;

FIG. 4 is a perspective view of a core nut of a blind fastenerconstructed in accordance with the teachings of the present disclosure;

FIG. 5 is another perspective view of the core nut of FIG. 4;

FIG. 6 is a side view of the core nut of FIG. 4;

FIG. 7 is a cross-sectional view of the core nut, taken along line A-Aof FIG. 6;

FIG. 8 is a partial perspective view of the core nut of FIG. 4;

FIG. 9 is another partial perspective view of the core nut of FIG. 4;

FIG. 10 is a cross-sectional view of a blind fastener in an installedstate before portions of a core bolt and a core nut break off;

FIG. 11 is an end view of the blind fastener of FIG. 10;

FIG. 12 is an enlarged view of portion B of FIG. 10;

FIG. 13 is a cutaway view of a blind fastener in a final installed stateafter portions of a core bolt and a core nut break off;

FIG. 14 is a perspective view of a blind fastener after portions of acore nut and core bolt break off to form flush surfaces;

FIG. 15 is a cutaway view of the blinder fastener of FIG. 14;

FIG. 16 is a partial perspective view of the blinder fastener of FIG.14, wherein a core bolt is removed to show a threaded part of the corenut;

FIG. 17 is a perspective view of a plurality blind fasteners installedin a panel;

FIG. 18 is a perspective view of a blind fastener constructed inaccordance with a second form of the present disclosure;

FIG. 19 is another perspective view of a blind fastener of FIG. 18;

FIG. 20 is a perspective view of a blind fastener installed in panelsand constructed in accordance with a second form of the presentdisclosure;

FIG. 21A is a cross-sectional view of a blind fastener constructed inaccordance with a second form of the present disclosure;

FIG. 21B is an enlarged view of portion C of FIG. 21A;

FIG. 21C is a stress diagram of a portion of the fastener under aload/force;

FIG. 22A to 22E depict a series of breakage and deformation action ofthe blind fastener; and

FIG. 23 is a perspective view of a fastener in its initial installationstage and a fastener in its final installation stage.

Corresponding reference numerals indicate corresponding parts throughoutthe several views of the drawings.

DETAILED DESCRIPTION

The following description is merely exemplary in nature and is notintended to limit the present disclosure, application, or uses.

Referring to the FIGS. 1 to 3, a blind fastener 30 according to a firstform of the present disclosure is configured to connect a plurality ofpanels including a front panel 1 and a rear panel 2, and is configuredto be inserted into a countersunk hole 31 of the front and rear panels 1and 2. The fastener 30 includes a core bolt 3, a core nut 5 surroundinga middle portion of the core bolt 3, and a sleeve 4 surrounding a rearportion of the core bolt 3. The rear portion of the core bolt 3 definesa smooth section 32, and the front portion and the middle portion of thecore bolt 3 define a threaded section 34. A core bolt stop shoulder 10is defined between the smooth section 32 and the threaded section 34 ofthe core bolt 3. An end of the front portion defines a tool engagementsection configured to be gripped by a tool (not shown) so that the toolcan impart torque on the core bolt 3 to rotate the core bolt 3. In theexample provided, the tool engagement section of the core bolt 3includes a plurality of flat surfaces 23.

The smooth section 32 of the core bolt 3 has an outer peripheral surfaceconstituting a smooth rotational surface 12. The threaded section 34 ofthe core bolt 3 includes outer threads and a break-off notch 14 adjacentto the core bolt stop shoulder 10. The break-off notch 14 constitutes aweaker point at the core bolt 3 so that the core bolt 3 can be broken atthe break-off notch 14 by applying a torsional force (for a thread typefastener) or a pulling force (for a pull type fastener) to the core bolt3 after the fastener 30 is completely installed in the panels 1 and 2.In other words, the break-off notch 14 forms a frangible portion of thecore bolt 3. In the example provided, the break-off notch 14 is disposedwithin the threaded section 34 such that the threads extend in bothaxial directions relative to the break-off notch 14 and the break-offnotch 14 is configured so that it does not interfere with the threads ofthe threaded section 34 turning through mating threads even when thearea with the break-off notch 14 is threaded through the mating threads.

The core bolt 3 further includes a core bolt head 9 at a rear end of thecore bolt 3. The core bolt head 9 is disposed outside the sleeve 4 anddefines a recess 8 that facilitates a removal process of the fastener 30at the end of the product lifecycle by drilling out the fastener 30. Thecore bolt head 9 extends radially outward of the rear end of the sleeve4 to overlap the rear end of the sleeve 4.

The sleeve 4 has a tubular configuration and includes an innerperipheral surface 11 constituting a smooth rotational surface 11 incontact with the smooth rotational surface 12 of the core bolt 3. Thesmooth rotational surface 12 of the core bolt 3 and the smoothrotational surface 11 of the sleeve 4 function as rotating bearingsurfaces, which may be lubricated, when a front end 13 of the core bolt3 is tightened or loosened to move the core bolt 3 relative to the corenut 5.

Referring to FIGS. 4 to 6, the core nut 5 has a tubular configurationand includes a main body 40, a nut head 46 and a second tool engagementsection 42 having a larger outside diameter than the main body 40. Themain body 40 includes a cylindrical portion 44 and a ramped portion 45.The second tool engagement section 42 includes a cylindrical portion 48and a flange 50. A peripheral groove 52 is disposed between the nut head46 and the second tool engagement section 42 to form a frangible portionof the core nut 5. The cylindrical portion 44 of the main body 40further includes a plurality of dimples 58 at the front end. The rampedportion 45 includes an outward facing ramp surface 22 and a plurality ofrecess grooves 60 are formed on the ramp surface 22. The ramp surface 22and the plurality of recess grooves 60 facilitate a slide-over action ofthe sleeve 4 during installation of the fastener 30, which will bedescribed in more detail below. The second tool engagement section 42 isconfigured to be gripped by a tool during installation of the fastener30 to prevent the core nut 5 from rotating when the core bolt 3 isscrewed into or out of the core nut 5.

As clearly shown in FIG. 1, during installation of the fastener 30, thecylindrical portion 44 of the main body 40 and the nut head 46 aredisposed in the countersunk hole 13 of the front and rear panels 1 and3, with the nut head 46 in the countersunk portion of the hole 13 thatis within the front panel 1. The second tool engagement section 42including the cylindrical portion 48 and the flange 50 is disposedoutside the front and rear panels 1 and 3. The peripheral groove 52between the nut head 46 and the second tool engagement section 42functions as a break-off groove to allow the second tool engagementsection 42 to break off after the fastener 30 is completely installed,which will be described in more detail below.

Referring to FIGS. 7 to 9, the core nut 5 defines a counter bore 20including a smooth part 72, a threaded part 74, and an enlarged part 76.The enlarged part 76 is formed in the tool engagement section 42 of thecore nut 5 and has a larger inside diameter than the threaded part 74and the smooth part 72. In the example provided, the threaded part 74 ispositioned such that the internal threads extend axially through aportion of the nut head 46 and the cylindrical portion 44. The smoothpart 72 has a larger inside diameter than the threaded part 74 such thata core nut stop shoulder 15 is defined between the smooth part 72 andthe threaded part 74. The nut head 46 includes a conical surface 80 andan end surface 82 and is configured to be disposed in the countersunkhole 31 to retain the core nut 5 in one axial direction after the secondtool engagement section 42 breaks off. In the example provided, the nuthead 46 includes an outer cylindrical surface 83 (labeled in FIG. 12)adjacent to the conical surface 80. The groove 52 is recessed radiallyinward from the outer cylindrical surface 83 and the outer surface ofthe cylindrical portion 48 of the second tool engagement section 42.

Referring back to FIG. 1, during initial installation, the threaded part74 of the core nut 5 is threadedly engaged to the threaded section 34 ofthe core bolt 3. The break-off notch 14 of the core bolt 3 is disposedaway from the front surface 16 of the front panel 1, and the core boltstop shoulder 10 is disposed away from the core nut stop shoulder 15 ofthe core nut 5. The end surface 82 of the nut head 46 of the core nut 5and the break-off groove 52 of the core nut 5 are flush with the frontsurface 16 of the front panel 1.

Referring to FIGS. 1 and 10 to 12, by tightening the core bolt front end13, the core bolt head 9 is moved towards the core nut 5 until the corebolt stop shoulder 10 comes to a positive stop and abuts against thecore nut stop shoulder 15. Concurrently, the break-off notch 14 is movedto be flush with the front surface 16 of the front panel 1 and the endsurface 82 of the nut head 46. The break-off groove 52 is also flushwith the front surface 16 of the front panel 1. It is understood thatthe core bolt stop shoulder 10 and the core nut stop shoulder 15 areoptional and do not have to be present in the blind fastener 30 tocreate flush surfaces in the blind fastener 30 after the blind faster iscompletely installed.

As the core bolt 3 is tightened and the core bolt head 9 is moved towardthe core nut 5, the sleeve 4 is compressed and deformed by the core bolthead 9. The sleeve 4 expands and covers the ramp surface 22 of the corenut 5 and slides toward the rear surface of the rear panel 2. Radialcore nut serrations 19 may be formed on the core nut 5 to prevent thesleeve 4 from freely rotating during the initial installation. Beforecomplete installation of the fastener 30, the break-off notch 14 and theexternal threads of the core bolt 3 are disposed inside the counter bore20 of the core nut 5 and are prevented against damage by any externalforce. The break-off notch 14 and the external threads of the core bolt3 may be lubricated or sealants may be applied.

When the sleeve 4 is slid over the ramp surface 22 of the core nut 5,the sleeve 4 is deformed, forming a bulb portion 21. The bulb portion 21may have a convex configuration depending on the total grip length ofthe panels 1 and 2. The size of the bulb portion 21 may be varieddepending on the grip length, which is equal to the thickness of thepanels to be connected. The recess grooves 60 formed on the ramp surface22 of the core bolt 3 may facilitate the sleeve front to cave into therecess grooves 60 to avoid a defective tulip condition, i.e., the sleevefront flaring out. As shown in FIG. 1, the sleeve 4 may have a counterbore 17 having an enlarged hole adjacent to the core nut 5. The enlargedhole of the counter bore 17 also facilitates the slide-over action ofthe sleeve 4 over the ramp surface 22 of the core nut 5.

As clearly shown in FIG. 10 and with continued reference to FIGS. 1 and7, an internal positive stop is provided between the core bolt stopshoulder 10 and the core nut stop shoulder 15. Therefore, the bulbportion 21, in a tensile linear stage, is not in a fully compressedposition. The bulb portion 21 can provide a higher tensile preload inkeeping the panels 1, 2 together in a high frequency of vibration.Moreover, the internal positive stop by the core bolt stop shoulder 10and the core nut stop shoulder 15 allows the fastener 30 to reach adesired panel thickness, preferably an average distance between maximumand minimum panel thicknesses. This positive stop can also preciselyregulate the break-off position of the core bolt 3 and the core nut 5 ofthe fastener 10, to ensure it would not exceed the tolerances ofprotrusion or recess of the mounting panel 1 or the countersunk head.

Referring to FIGS. 1, 7, and 13, after the fastener 30 is installed inthe panels 1 and 3, by continuing to apply a torsional force on the corebolt 3, the threaded front portion of the core bolt 3 is broken at thebreak-off notch 14. An end surface of the core bolt 3 is formed andbecomes flush with the front surface 16 of the panel 1. Also, byapplying torsional force on the core nut 5, the second tool engagementsection 42 breaks off at the break-off groove 52. The end surface 82 ofthe main body 40 is exposed and is flush with the front surface 16 ofthe front panel 1.

Referring to FIGS. 14 to 16, the fastener 30 is shown to be in acompletely installed state with the second tool engagement section 42 ofthe core nut 5 and the front portion of the core bolt 3 broken off. Thepanels 1 and 3 being connected are not shown for clarity. As shown, theend surface 82 of the main body 40 becomes the outermost end of thefastener 30 in its completely installed state, and the end surface 82 isflush with a surface of the core bolt 3 that is formed after the frontportion of core bolt 3 breaks off at the break-off notch 14. In FIG. 16,the core bolt 3 is removed to show the threaded part 74 of the core nut5.

Referring to FIG. 17, a plurality of countersunk holes 31 are shown tobe formed through a front panel 1. Fasteners are installed in some ofthe countersunk holes 31. After the front portions of the core bolts 3and the second tool engagement section 42 of the core nuts 5 break off,flat end surfaces are formed on the core bolts 3 to be flush with thefront surface 16 of the front panel 1. Also, the end surfaces 82 of thecore nuts 5, which are preformed and configured to be flush with thefront surface 16 of the front panel 1 during installation, are exposed.In one countersunk hole 31 of FIG. 17, the front portion of the corebolt 3 breaks off but the second tool engagement section 42 of the corenut 5 remains on the fastener 30, resulting in an uneven appearance.

Referring to FIGS. 18 to 20, a blind fastener 90 according to a secondform of the present disclosure has a structure similar to the blindfastener 30 of the first form except that the sleeve and the core nutare integrally formed as a one-piece component. In the following, likeelements are indicated by like reference numerals and the detaileddescription thereof is omitted herein for clarity.

More specifically, the blind fastener 90 includes a core bolt 3 and anouter member 92 surrounding the core bolt 3. The core bolt 3 includes acore bolt head 9 at a rear end. The core bolt 3 is inserted into theouter member 92 with the core bolt head 9 disposed outside the outermember 92 and abutting against a rear end of the outer member 92.

Referring to FIG. 21A, the outer member 92 includes a sleeve 96 and anut 98 connected to the sleeve 96 at a breakable interface 100. Thesleeve 96 and the nut 98 are integrally formed. The sleeve 96 isstructurally similar to the sleeve 4 of the first form. The nut 98 isstructurally similar to the core nut 5 of the first form. Therefore, thedetailed description of the sleeve 96 and the nut 98 is omitted hereinfor clarity.

Referring to FIGS. 21A and 21B, when the core bolt 3 is rotated relativeto the one-piece outer member 92 to screw the core bolt 3 out of theouter member 92 in a forward direction Y, a force is applied on theouter member 92 and causes the breakable interface 100 between thesleeve 96 and the nut 98 to break, thereby separating the sleeve 96 fromthe nut 98. The breakable interface 100 may be configured to break at astress level of 20-30 in-lbs, though other configurations can be used.After the breakable interface 100 breaks, the blind fastener 90 isstructurally similar to the blind fastener 30 of the first form. As theforce continues to be applied on the core bolt 3, the sleeve 96 that isdisposed between the nut 98 and the core bolt head 9 slides over the nut98 and is compressed and deformed to form a bulb portion. After thebreakable interface 100 breaks, the blind fastener 90 has the sameoperation as that of the blind fastener 30 and thus the detaileddescription thereof is omitted herein.

Referring to FIGS. 21A and 21C, a stress diagram of a part of the blindfastener 90 and the panels 1 and 2 is shown. Under the same load/force,the panels 1 and 2 and the nut 98 of the outer member 92 have the loweststress, whereas the sleeve 96 of the outer member 92 has the higheststress. The core bolt 2 has a stress level between the nut 98 and thesleeve 96 of the outer member 92 under the same load/force. Therefore,using the blind fastener 90 of the present disclosure, the panels 1 and2 will not be undesirably deformed by excessive force because the corebolt 3 will break before the panels 1 and 2 are deformed. On the otherhand, the sleeve 96 of the outer member 92 has the highest stress underthe same load/force and is first to be deformed, making the blindfastener 90 relatively easily installed in the panels 1 and 2.

Referring to FIGS. 22A to 22E, a series of breakage and deformationactions of the blind fastener 90 caused by a torsional force applied tothe core bolt 3 are described. FIG. 22A shows the position of the outermember 92 relative to the core bolt 3 during initial installation, wherethe sleeve 96 remains connected to the nut 98 of the outer member 92.The sleeve 96 includes a front end 102 and a rear end 104. The panels 1and 2 may be made of aluminum. The core bolt 3 may be made of A286stainless steel and can withstand a stress of 177-196 ksi. The differentportions of the outer member 92, due to its varied thickness, canwithstand different stress. The front end 102 of the sleeve 96 of theouter member 92 can withstand a stress of 75-78 ksi. The rear end 104 ofthe sleeve 96 of the outer member 92 can withstand a stress of 130-140ksi. The nut 98 of the outer member 92 can withstand a stress of 177-196ksi.

FIG. 22B shows the breakable interface 100 between the sleeve 96 and thenut 98 is the first break point to break during installation of theblind fastener 90, causing the sleeve 96 to be separated from the nut98. The breakable interface 100 can withstand a stress of 20-30 in-lbs.

Referring to FIG. 22C, after the sleeve 96 is separated from the nut 98,the sleeve 96, which is disposed between the core bolt head 9 and thenut 98, is compressed therebetween as the core bolt 3 (and consequentlythe core bolt head 9) continues to move in the forward direction Y. As aresult, a bulb portion is created in the sleeve 96.

Referring to FIG. 22D, as the core bolt 3 continues to be moved in theforward direction Y, the sleeve 96 slides over the nut 98 and the corebolt stop shoulder 10 is moved toward the stop shoulder 15 of the nut 98until the two stop shoulders 10 and 15 contact each other. After thestop shoulder 10 of the core bolt 3 abuts against the stop shoulder 15of the nut 98, continuing application of force will cause the core bolt3 to break at the break-off notch 14, which is configured to break at astress level of 40-45 in-lbs. It is understood that the stop shoulders10 and 15 do not have to be present to make the core bolt 3 breaks atthe break-off notch 14.

Referring to FIG. 22E, after the front portion of the core bolt 3 breaksoff at the break-off notch 14, a second tool engagement section 42 ofthe nut 98 may break off at the break-off groove 52, which is configuredto break at a stress level of 75-95 in-lbs.

Therefore, the blind fastener 90 of the second form may initially be atwo-piece blind fastener that breaks at three different break points atdifferent stages of installation and results in three separate piecesholding the panels 1 and 2 together. During initial installation, thesleeve 96 is separated from the nut 98 of the outer member 92 at thebreakable interface 100 at a stress level of 20-30 in-lbs. After thesleeve 96 is deformed to have a bulb portion and the first panel 1 andthe second panel 2 are clamped between the second tool engagementsection 42 of the nut 98 and the bulb portion of the sleeve 96, thefront portion of the core bolt 3 can break off at a stress level of40-45 in-lbs, creating a flush surface 108 at the core bolt 3.Thereafter, the second tool engagement section 42 of the nut 98 canbreak at the break-off groove at a stress level of 75-90 in-lbs, therebyexposing the end surface 82 of the nut 98, which is another flushsurface created by blind fastener 90.

Referring to FIG. 23, the blind fastener 90 on the right hand side isshown to be in its initial installation position where the sleeve 96 hasyet to be deformed to form a bulb portion, whereas the blind fastener 90on the left hand side is shown to be in its final installation positionwhere the front portion of the core bolt 3 breaks off at the break-offnotch 14 to form a flush surface 108 on the core bolt 3 and the secondtool engagement section 42 of the nut 98 breaks off at the break-offgroove 52 to form a flush surface 82 on the nut 98 of the outer member92.

A typical fastener (not shown) may result in a break-off end beingdisposed inside the countersunk end of the core nut 5 or protrudingoutside the countersunk end of the core nut 5. Therefore, the secondarywork to grind, shave, sand and fill in the front side of the fastener istypically required. This process requires a lot of labor time and costs,thereby increasing manufacturing costs, which is particularlysignificant in the aerospace industry where approximately 8,000 to10,000 fasteners are used in each commercial aircraft.

In contrast, the fastener of the present disclosure is configured tocreate a flush surface with the front surface of the front panel afterthe blind fastener is completely installed. Therefore, the fasteners 30,90 of the present disclosure can shorten the manufacturing time andreduce the manufacturing costs since no secondary work is required totreat the front surface of the fastener.

The fastener of the present disclosure also provides an aesthetic flushmounting and optimizes aerodynamic appearance. The fastener of thepresent disclosure provides a higher preload (compression pressure ofthe sandwiched mounting panels) with a minimum 50% of the tensilestrength. Current blind fasteners on the market are fullycompressed/deformed, which offers less preload value. Higher preloadvalues are significantly beneficial in handling the high frequencyvibrations of aircraft, in both threaded-type blind fasteners andpull-type blind rivets. The fastener of the present disclosure preventsover torque/pulling that creates a lower preload value, and defectiveinstallation due to a tulip configuration.

It should be noted that the disclosure is not limited to the formdescribed and illustrated as examples. A large variety of modificationshave been described and more are part of the knowledge of the personskilled in the art. These and further modifications as well as anyreplacement by technical equivalents may be added to the description andfigures, without leaving the scope of the protection of the disclosureand of the present patent.

As used herein, the phrase at least one of A, B, and C should beconstrued to mean a logical (A OR B OR C), using a non-exclusive logicalOR, and should not be construed to mean “at least one of A, at least oneof B, and at least one of C.

Unless otherwise expressly indicated, all numerical values indicatingmechanical/thermal properties, compositional percentages, dimensionsand/or tolerances, or other characteristics are to be understood asmodified by the word “about” or “approximately” in describing the scopeof the present disclosure. This modification is desired for variousreasons including industrial practice, manufacturing technology, andtesting capability.

The terminology used herein is for the purpose of describing particularexample forms only and is not intended to be limiting. The singularforms “a,” “an,” and “the” may be intended to include the plural formsas well, unless the context clearly indicates otherwise. The terms“including,” and “having,” are inclusive and therefore specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof. The method steps, processes, andoperations described herein are not to be construed as necessarilyrequiring their performance in the particular order discussed orillustrated, unless specifically identified as an order of performance.It is also to be understood that additional or alternative steps may beemployed.

The description of the disclosure is merely exemplary in nature and,thus, examples that do not depart from the substance of the disclosureare intended to be within the scope of the disclosure. Such examples arenot to be regarded as a departure from the spirit and scope of thedisclosure. The broad teachings of the disclosure can be implemented ina variety of forms. Therefore, while this disclosure includes particularexamples, the true scope of the disclosure should not be so limitedsince other modifications will become apparent upon a study of thedrawings, the specification, and the following claims.

What is claimed is:
 1. A blind fastener for connecting a plurality ofpanels, comprising: a core bolt; and a core nut surrounding the corebolt, the core nut including a main body, a nut head, and a toolengagement section, the nut head having a larger outside diameter thanthe main body, wherein the core nut defines a frangible portion betweenthe nut head and the tool engagement section, the frangible portionconfigured to allow the tool engagement section to break off from thenut head when a torque applied to the tool engagement section is greaterthan a threshold torque after the blind fastener is installed in theplurality of panels.
 2. The blind fastener according to claim 1, whereinthe frangible portion is configured to break off from the nut head suchthat an end surface of the nut head is flush with or recessed from afront one of the plurality of panels.
 3. The blind fastener according toclaim 1, wherein a radially outward facing surface of the main body ofthe core nut includes a plurality of dimples.
 4. The blind fasteneraccording to claim 1, wherein the blind fastener consists of two pieceswhen in a pre-installed condition, the core bolt being a first one ofthe two pieces and the core nut being a second one of the two pieces. 5.The blind fastener according to claim 1, wherein the frangible portionincludes a break-off groove that extends along an entire periphery ofthe core nut.
 6. The blind fastener according to claim 5, wherein thenut head defines a first cylindrical surface and the tool engagementportion defines a second cylindrical surface, wherein the break-offgroove is recessed radially inward from the first and second cylindricalsurfaces.
 7. The blind fastener according to claim 1, wherein the corenut defines a counter bore having an enlarged part and a threaded part,the enlarged part being formed in the tool engagement portion and havinga larger inside diameter than the threaded part, wherein the threadedpart is disposed in at least one of the nut head and an end of the mainbody proximal to the nut head.
 8. The blind fastener according to claim1, wherein the main body defines a counter bore having a smooth part anda threaded part, the smooth part having an inside diameter larger thanthe threaded part such that a nut stop shoulder is defined between thesmooth part and the threaded part, wherein the core bolt defines a boltstop shoulder that is configured to abut the nut stop shoulder when afrangible portion of the core bolt is aligned with or recessed from anouter surface of a front one of the plurality of panels.
 9. The blindfastener according to claim 1, wherein the core nut further includes asleeve coupled to an end of the main body that is distal to the nuthead, wherein the core bolt includes a bolt head engaged with an end ofthe sleeve that is distal to the main body, and wherein the main bodyincludes a ramp surface configured to slidingly engage an inner surfaceof the sleeve.
 10. The blind fastener according to claim 9, wherein thebolt head includes a central recess facing axially outward from the corebolt.
 11. The blind fastener according to claim 9, wherein the sleeve iscoupled to the end of the main body by a second frangible section of thecore nut.
 12. The blind fastener according to claim 9, wherein the rampsurface includes a plurality of grooves.
 13. The blind fasteneraccording to claim 1, wherein the nut head defines a conical surface andan end surface, the conical surface being configured to abut acountersunk surface of a front panel of the plurality of panels, the endsurface configured to be flush with or recessed from a forward surfaceof the front panel when the conical surface abuts the countersunksurface.
 14. A blind fastener for connecting a plurality of panels,comprising: a core bolt including a first tool engagement section, athreaded section and a bolt head, the threaded section including a firstfrangible portion configured to break the threaded section when a torqueapplied to the first handling section exceeds a first threshold torque,the threaded section being axially between the first tool engagementsection and the bolt head; and a core nut including a main body, a nuthead, a second frangible portion, and a second tool engagement section,the second frangible portion frangibly coupling the second toolengagement section to the nut head and configured to break the secondtool engagement section off from the nut head when a torque applied tothe second tool engagement section exceeds a second threshold torque,the nut head being axially between the second frangible section and themain body, wherein at least one of the main body and the nut headdefines internal threads configured to engage the threaded section ofthe core bolt, and the bolt head is configured to engage an end of thecore nut that is opposite the nut head.
 15. The blind fastener accordingto claim 14, wherein the main body includes a ramp section, a sleevesection, and a third frangible section that frangibly couples the sleevesection to the ramp section.
 16. The blind fastener according to claim14, wherein the core nut further includes a sleeve section and the mainbody includes a ramp surface configured to slidably engage an innersurface of the sleeve section.
 17. The blind fastener according to claim14, wherein the nut head defines a conical surface and an end surface,the conical surface being configured to abut a countersunk surface of afront panel of the plurality of panels, the end surface configured to beflush with or recessed from a forward surface of the front panel whenthe conical surface abuts the countersunk surface.
 18. The blindfastener according to claim 16, wherein the main body defines a counterbore having a smooth part and a threaded part, the threaded partdefining the internal threads, the smooth part having an inside diameterlarger than a major diameter of the internal threads of the threadedpart such that a nut stop shoulder is defined between the smooth partand the threaded part, wherein the core bolt defines a bolt stopshoulder that is configured to abut the nut stop shoulder when afrangible portion of the core bolt is aligned with or recessed from anouter surface of a front one of the plurality of panels.
 19. An assemblycomprising: a front panel defining a front surface; a rear paneldefining a rear surface; a blind fastener inserted into holes of thefront and rear panels for connecting the front and rear panels together,the blind fastener comprising: a core bolt received in the holes of thefront and rear panels and defining a break-off notch and a bolt head;and a core nut surrounding the core bolt, the core nut including a mainbody, a nut head, a sleeve, and a tool engagement section, the main bodyextending within the holes of the front and rear panels, the nut headdisposed in a counterbore or countersink of the hole of the front panel,the tool engagement section disposed outside the front and rear panelsand adjacent to the front panel, the sleeve being coupled to an end ofthe main body that is opposite the nut head and disposed outside andadjacent to the rear panel, the sleeve being configured to be deformedby axial force imparted by the bolt head moving toward the panels, thecore nut further defining a break-off groove that frangibly couples thetool engagement section to the nut head, wherein the break-off groove ofthe core nut and the break-off notch of the core bolt are flush with orrecessed from the front surface of the front panel when the blindfastener is fully installed.
 20. The assembly according to claim 19,wherein the nut head of the core nut includes an end surface that isflush with the front surface of the front panel.